Evolution of Reaction Norms

Abstract

Phenotypic variance is a function of genetic variability, environmental variation, and the ways in which genetic and environmental variation interact, i.e., VG×E. Reaction norms are a means of conceptually, graphically, and mathematically describing this total variance and are a powerful tool for decomposing it into its constituent parts (i.e., nature, nurture, and, critically, their interaction). A reaction norm is defined as the range of phenotypes expressed by a genotype along an environmental gradient. It is represented by a linear or nonlinear function which describes the value of a phenotypic trait for a particular genotype or group of genotypes in different environments. As such, it is closely related to the concept of phenotypic plasticity, which can be represented by a reaction norm with a non-zero slope (i.e., the phenotype varies with respect to the environment). While the term (which originated as Reaktionsnorm) has been in use for over one hundred years, there has been some debate about the most appropriate way to describe it mathematically. Nonetheless, there is general consensus that a reaction norm has multiple properties, each of which can be the target of selection. Reaction norms are typically described as consisting of: (1) an intercept, elevation, or offset, which describes the mean trait value across all environments, (2) a slope, which quantifies the degree of trait plasticity, and (3) shape or curvature (e.g., linear, quadratic, monotonic). Evidence that trait means and plasticities can evolve separately underscores the necessity of applying a reaction norm framework for studying ecological and evolutionary responses to the environment, because measuring phenotypes in a single environmental context does not necessarily reflect their relative values or diversities in a different context. These contextual differences are particularly important in a world of rapid anthropogenic change and increasing environmental variability. Therefore, in addition to being fundamental to ecological and evolutionary phenomena, reaction norm evolution is relevant for diverse biological fields, including behavior and psychology, conservation and natural resource management, global change biology, agriculture and breeding programs, and human health. Given that evolutionary change is defined by genetic change, we focus this article on variation among reaction norms from different genotypes (i.e., reaction norms that have potentially evolved to be divergent from one another) as well as the forces that promote and constrain reaction norm evolution. For an overview of the literature on plasticity itself (keeping in mind that reaction norms need not be plastic), see the separate Oxford Bibliographies in Evolutionary Biology article Phenotypic Plasticity.